Performance of a digital subscriber line (DSL) in terms of capacity depends on a number of factors such as attenuation and a noise environment. Performance of a DSL transmission system is impacted by crosstalk interference from one twisted line pair to another twisted line pair with the same binder and, to a lesser extent, twisted line pairs in neighboring binders.
Consequently, crosstalk interference may affect data rates across a number of twisted pair lines.
For instance two communication lines such as two VDSL2 lines which are collocated next to each other induce a signal in each other. Due to the induced crosstalk and noise from other sources in the surroundings of the communication line, the data transported on these lines may be affected or corrupted by the crosstalk and noise. By reducing the crosstalk induced on a communication line or compensating the crosstalk induced on a communication line, the amount of corrupted data may be reduced and the rate at which information can be reliably communicated is increased.
Each communication line is a possible disturber line which induces crosstalk in one or more victim lines. Moreover, in today's systems, the number of active communication lines may vary. Thus, the induced crosstalk varies as the number of active communication lines vary.
By transmitting test signals across all the lines, it is possible to determine the influence of each disturber line on the victim lines. The test signals can be characterized by the way in which power is allocated to one or more tones or frequencies. For instance, a test signal may be transmitted using a particular power level over a small frequency range. The victim line may notice this power in that frequency range and be able to determine the amplitude of that power. The amplitude of the induced influence of crosstalk on a particular line is a good reference to determine how strong particular crosstalk disturbers are into that victim or which frequencies or tones are susceptible to the crosstalk of certain crosstalk disturbers into that victim.
Precoding (also referred to as precompensation) techniques are based on transmitting an additional signal added to the data signal which is used to compensate the crosstalk on a victim line from external sources. Thus, instead of reducing the effect of crosstalk or avoiding crosstalk effects by configuring the communication line in an appropriate way, precoding can be used to compensate for the effects of crosstalk on a communication channel. Precoding techniques are based on crosstalk channel information that includes both amplitude and phase information. Such information can be obtained by processing measurements such as slicer error or signal-to-noise ratio (SNR).
A particular example of such measurements for precoding is the use of pilot sequences and error feedback. The use of pilot sequences in G.vector is described in “Self-FEXT cancellation (vectoring) for use with VDSL2 transceivers,” Series G: Transmission Systems and Media, Digital Systems and Networks, ITU G.993.5, April 2010, the entire contents of which is incorporated by reference.
Initialization of a vectored DSL session generally uses an estimation process to estimate crosstalk channel coupling between activating lines and already active lines.